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| United States Patent |
5,658,464
|
|
Hann
, et al.
|
August 19, 1997
|
Method of inhibiting sulfate scale in aqueous systems using poly(amino
acids)
Abstract
The present invention provides a method for inhibiting the formation of
metal sulfate scale in an aqueous system. Metal sulfate scale formation is
inhibited by adding an effective amount of one or more poly(amino acids)
and one or more inorganic phosphates to the aqueous system. The poly(amino
acids) are a reaction product formed from at least one compound selected
from amino acids, amic acids, ammonium salts of monoethylenically
unsaturated dicarboxylic acids, and ammonium salts of
hydroxypolycarboxylic acids. The present invention also provides a method
of inhibiting the formation of metal sulfate scale by adding an effective
amount of a poly(amino acid) which contains tyrosine, tryptophan,
histidine, arginine or combinations thereof.
| Inventors:
|
Hann; William Mathis (Gwynedd, PA);
Paik; Yi Hyon (Princeton, NJ);
Robertson; Susan Tabb (Ambler, PA);
Swift; Graham (Blue Bell, PA)
|
| Assignee:
|
Rohm and Haas Company (Philadelphia, PA)
|
| Appl. No.:
|
706142 |
| Filed:
|
September 20, 1996 |
| Current U.S. Class: |
210/697; 210/698; 210/699; 252/180; 252/181 |
| Intern'l Class: |
C02F 005/14 |
| Field of Search: |
210/697-701
252/180,181
|
References Cited
U.S. Patent Documents
| 3846380 | Nov., 1974 | Fujimoto et al. | 260/784.
|
| 4534881 | Aug., 1985 | Sikes et al. | 210/698.
|
| 4575425 | Mar., 1986 | Boffardi et al. | 210/697.
|
| 4587021 | May., 1986 | Wheeler et al. | 210/698.
|
| 4590260 | May., 1986 | Harada et al. | 528/328.
|
| 4603006 | Jul., 1986 | Sikes et al. | 252/180.
|
| 4804476 | Feb., 1989 | Sinkovitz et al. | 210/697.
|
| 4839461 | Jun., 1989 | Boehmke | 528/363.
|
| 4868287 | Sep., 1989 | Sikes et al. | 530/324.
|
| 4980433 | Dec., 1990 | Chen et al. | 526/240.
|
| 5051401 | Sep., 1991 | Sikes | 514/7.
|
| 5057597 | Oct., 1991 | Koskan | 528/328.
|
| 5116513 | May., 1992 | Koskan et al. | 210/698.
|
| 5152902 | Oct., 1992 | Koskan et al. | 210/698.
|
| 5204099 | Apr., 1993 | Barbier et al. | 424/401.
|
| 5260272 | Nov., 1993 | Donachy et al. | 524/12.
|
| 5284512 | Feb., 1994 | Koskan et al. | 106/416.
|
| 5302293 | Apr., 1994 | Kaplan et al. | 210/701.
|
| 5306429 | Apr., 1994 | Wood et al. | 210/698.
|
| 5328690 | Jul., 1994 | Sikes | 424/401.
|
| 5332505 | Jul., 1994 | Carey et al. | 210/697.
|
| 5408028 | Apr., 1995 | Wood et al. | 528/328.
|
| 5457176 | Oct., 1995 | Adler et al. | 528/328.
|
| 5506335 | Apr., 1996 | Uhr et al. | 528/322.
|
| 5523023 | Jun., 1996 | Kleinstuck et al. | 252/542.
|
| 5525257 | Jun., 1996 | Kleinstuck et al. | 252/181.
|
| 5531934 | Jul., 1996 | Freeman et al. | 252/390.
|
| Foreign Patent Documents |
| 692459 | Jan., 1996 | EP.
| |
Other References
S. Sarig, et al., "The Use of Polymers for Retardation of Scale Formation",
National Council for Research and Development, 150-157 (1977).
S. Sarig, et al., "Selection of Threshold Agents for Calcium Sulfate Scale
Control on the Basis of Chemical Structure", Desalination 17(2):215-229
(1975).
Inhibition Of Barium Sulfate Precipitation: Effects Of Additives, Solution
pH, And Supersaturation, Water Treatment, 9 (1994) 47-56, Zahid Amjad, The
BF Goodrich Company, Specialties Polymer and Chemicals Division,
Brecksville, OH, U.S.
|
Primary Examiner: Hruskoci; Peter A.
Attorney, Agent or Firm: Hild; Kimberly R.
Parent Case Text
This application is a continuation of application Ser. No. 08/304,056,
filed Sep. 12, 1994, now abandoned.
Claims
We claim:
1. A method of inhibiting scale formation, comprising: adding to an aqueous
system an effective amount of one or more poly(amino acids) and from 0.5
to 100 milligrams per liter of one or more inorganic phosphates; wherein
the poly(amino acids) comprise a reaction product of at least one compound
selected from the group consisting of: glycine, alanine, valine, leucine,
isoleucine, phenylalanine, tyrosine, tryptophan, serine, threonine,
aspartic acid, glutamic acid, asparagine, glutamine, lysine, arginine,
histidine, .beta.-alanine, phosphoserine, hydroxylysine, 4-aminobutyric
acid, maleamic acid, ammonium salts of maleic acid, and ammonium salts of
malic acid, and combinations thereof; and wherein the scale is metal
sulfate scale.
2. The method of claim 1, wherein the reaction product further comprises
one or more optional additional monomers selected from the group
consisting of carboxylic acids, hydroxycarboxylic acids, alcohols,
alkoxylated alcohols, amines, alkoxylated amines, lactones, lactams, and
combinations thereof.
3. The method of claim 1, wherein: the poly(amino acids) are copolymers of
amino acids.
4. The method of claim 3, wherein: the copolymers of amino acids comprise a
reaction product of at least two different compounds independently
selected from the group consisting of glycine, alanine, valine, leucine,
isoleucine, phenylalanine, tyrosine, tryptophan, serine, threonine,
aspartic acid, glutamic acid, asparagine, glutamine, lysine, arginine,
histidine, .beta.-alanine, 4-aminobutyric acid, maleamic acid, ammonium
salts of maleic acid, and ammonium salts of malic acid, and combinations
thereof and wherein the copolymers contain at least two different types of
repeating units.
5. The method of claim 1, wherein: the poly(amino acids) are homopolymers
of amino acids.
6. The method of claim 5, wherein: the homopolymers of amino acids comprise
a reaction product of at least one compound selected from the group
consisting of: aspartic acid, glutamic acid, asparagine, glutamine,
maleamic acid, ammonium salts of maleic acid, and ammonium salts of malic
acid.
7. The method of claim 5, wherein: the homopolymers of amino acids are
poly(aspartic acid).
8. The method of claim 1, wherein: the inorganic phosphates are
pyrophosphates.
9. The method of claim 1, wherein: the poly(amino acids) comprise a
reaction product of at least one compound selected from the group
consisting of: glycine, alanine, valine, leucine, isoleucine,
phenylalanine, tyrosine, tryptophan, serine, threonine, aspartic acid,
glutamic acid, asparagine, glutamine, lysine, arginine, histidine,
.beta.-alanine, 4-aminobutyric acid, maleamic acid, ammonium salts of
maleic acid, and ammonium salts of malic acid, and combinations thereof.
10. The method of claim 1, wherein the poly(amino acids) are added to the
aqueous system at a concentration of greater than 0.1 milligrams per
liter.
11. A method of inhibiting scale formation, comprising: adding to an
aqueous system an effective amount of one or more poly(amino acids),
wherein the poly(amino acids) comprise a reaction product of from 1 to 60
mole percent of at least one first compound selected from the group
consisting of histidine, arginine, tyrosine, tryptophan, and combinations
thereof and one or more second compounds selected from the group
consisting of: aspartic acid, glutamic acid, asparagine, glutamine,
maleamic acid, ammonium salts of maleic acid, ammonium salts of malic
acid, and combinations thereof; and wherein the scale is metal sulfate
scale.
12. The method of claim 11, wherein the first compound is histidine.
13. The method of claim 11, wherein the poly(amino acids) are copolymers of
aspartic acid and histidine.
14. The method of claim 11, wherein: the one or more poly(amino acids) are
added to the aqueous system at a concentration of from 1 milligram per
liter to 5000 milligrams per liter.
Description
BACKGROUND
The present invention relates to a method of inhibiting metal sulfate scale
formation in an aqueous system. More particularly, the invention is
directed to the use of certain poly(amino acids) to inhibit scale
formation of metal sulfates. The poly(amino acids) are preferably added
with at least one inorganic phosphate to the aqueous system.
Metal sulfate scale formation is a common problem in many aqueous systems.
The sulfate scale is formed in the aqueous systems, when cations, for
example alkaline earth metal ions, combine with sulfate ions. Metal
sulfate scale includes for example magnesium sulfate, calcium sulfate,
barium sulfate, strontium sulfate, radium sulfate, iron sulfate, and
manganese sulfate.
Aqueous systems where metal sulfate scale may form are for example in
cooling water systems, boilers, heat exchange equipment, reverse osmosis
equipment, sugar processing equipment, geothermal systems, oil and gas
production operations, flash evaporators, desalination plants, paper
making equipment, and steam power plants. Metal sulfate scale is
particularly found in paper making equipment, desalination plants, reverse
osmosis, and oil and gas production operations.
The metal sulfate scale forms on the surfaces of the aqueous systems
causing such problems for example as reduced heat transfer, plugged pipes,
and acceleration of corrosion. Once formed, the sulfate scale is difficult
to remove. Scraping, sand blasting, chipping, or chemical removal with
specially formulated cleaners may be required to remove the scale.
To prevent the formation of metal sulfate scale, scale inhibitors are added
to aqueous systems.
For example, in papermaking processes, scale inhibitors are added to
prevent excessive scale build-up in the equipment, such as in the headbox
and related piping, and on the fourdrinier wire. The metal sulfate scale
is formed from the interaction of the alkaline earth metals and sulfate
ions found in components used in the paper making process. For example,
barium and other alkaline earth metals are found in wood pulp, which is a
primary ingredient in the paper making process. Sources of sulfate ions in
the paper making process are, for example, from water or aluminum sulfate
which may be intentionally added to increase retention of other additives
during formation of paper.
Scale inhibitors are used in equipment to purify sea water or brackish
water in desalination plants. Such equipment includes for example reverse
osmosis equipment and distillation units. Because sea water or brackish
water contains both sulfate ions and alkaline earth metals, as the water
is purified, the metal sulfate concentrates and deposits on the equipment
surfaces to form scale. The scale formation can be a serious problem
causing the equipment to operate less efficiently leading to equipment
downtimes for cleaning and increased operating costs.
In oil production operations, the formation of metal sulfate scale is a
common and serious problem. Consequently scale inhibitors are commonly
used. The metal sulfate scale, most commonly barium sulfate and calcium
sulfate, typically form when alkaline earth metal ions from an aquifer or
from connate water, combine with water containing sulfate ions, such as
sea water or another aquifer previously unconnected to the first aquifer.
The metal sulfate deposits as scale in subterranean formations or in
topside equipment such as in pipes, valves, and pumps.
Scale inhibitors are used in various ways to prevent scale formation in oil
production operations. For example, in what is called a "squeeze
treatment", an aqueous solution, containing scale inhibitor, is forced
under pressure through the wells into the subterranean formation. The
scale inhibitors are believed to adsorb or precipitate onto the formation
and gradually desorb or resolubilize from the formation to inhibit scale
from depositing and prevent clogging of the formation with scale. The well
is then periodically resqueezed when the concentration of scale inhibitor
falls below an effective concentration for scale inhibition.
Scale inhibitors may also be fed into pipelines to prevent scale formation,
which would impede the transport of oil. Additionally, scale inhibitors
are used in secondary oil recovery operations, where pressurized water is
used to recover additional oil.
Scale inhibitors which have been used in aqueous systems to inhibit metal
sulfate scale formation are for example homopolymers and copolymers of
acrylic acid. More recently, there has been a trend to develop scale
inhibitors which have greater biodegradability. For example, U.S. Pat. No.
5,116,513 to Koskan, et al., discloses the use of a poly(amino acid),
poly(aspartic acid), having a molecular weight of 1000 to 5000 as a scale
inhibitor for calcium sulfate and barium sulfate. The poly(aspartic acid)
is produced by the thermal condensation of aspartic acid to form
polysuccinimide. The polysuccinimide is then hydrolyzed to form
poly(aspartic acid) which preferably is greater than 50 percent in
.beta.-form and less than 50 percent in .alpha.-form. The poly(aspartic
acid) is disclosed to be biodegradable.
The problem addressed by the present invention is to provide additives
which improve the performance of poly(amino acids) to inhibit metal
sulfate scale formation. The present invention also seeks to provide
certain poly(amino acids) which more effectively inhibit metal sulfate
scale formation.
We have found that adding one or more inorganic phosphates and one or more
poly(amino acids) to an aqueous system to inhibit metal sulfate scale
formation, is more effective than what would be expected by adding either
the phosphates or poly(amino acids) alone to the aqueous system.
STATEMENT OF THE INVENTION
We have discovered a method of inhibiting scale formation, comprising:
adding to an aqueous system an effective amount of one or more poly(amino
acids) and one or more inorganic phosphates; wherein the poly(amino acids)
comprise a reaction product of at least one compound selected from the
group consisting of: amino acids, amic acids, ammonium salts of
monoethylenically unsaturated dicarboxylic acids, and ammonium salts of
hydroxypolycarboxylic acids; and wherein the scale is metal sulfate scale.
In another embodiment of the present invention, we have discovered a method
of inhibiting scale formation, comprising: adding to an aqueous system an
effective amount of one or more poly(amino acids), wherein the poly(amino
acids) comprise a reaction product of at least one first compound selected
from the group consisting of histidine, arginine, tyrosine, tryptophan,
and combinations thereof; and wherein the scale is metal sulfate scale.
DETAILED DESCRIPTION
By "inhibiting metal sulfate scale formation" we mean that scale is
prevented from fouling or depositing on the surfaces of the aqueous
system. For example, two possible mechanisms in which metal sulfates can
be prevented from fouling or depositing on a surface are through 1)
inhibiting the precipitation or crystallization of the metal sulfates from
the water and 2) dispersing the metal sulfates once they have formed in
the water to prevent them from attaching to surfaces. These mechanisms are
presented as theory and are in no way meant to limit the present
invention.
We have discovered scale inhibitors useful in inhibiting the formation of
metal sulfates comprising one or more inorganic phosphates and one or more
poly(amino acids). The inorganic phosphates may be added to the aqueous
system together with the poly(amino acids), or added separately to the
aqueous system. If added separately, the inorganic phosphates may be added
before or after adding the poly(amino acids), or may be added
simultaneously with the poly(amino acids). Preferably, the inorganic
phosphates are added together with the poly(amino acids).
The total concentration of the one or more inorganic phosphates added with
the poly(amino acids) to the aqueous system is typically the minimum
amount needed to inhibit metal sulfate scale formation. Typically, the
concentration of the inorganic phosphates is at least 0.1 mg/l, more
preferably 0.5 to 100 mg/l, and most preferably 1 to 20 mg/l.
The total concentration of the poly(amino acids) added with the inorganic
phosphates to the aqueous system to be treated is an effective amount to
inhibit metal sulfate scale formation. Typically, the total concentration
of poly(amino acids) is greater than 0.1 mg/l, preferably from 1 to 1000
mg/l, and most preferably from 3 to 600 mg/l.
Inorganic phosphates which have been found to inhibit metal sulfate scale
formation with poly(amino acids) in aqueous systems include water soluble
and molecularly dehydrated phosphates. By "molecularly dehydrated
phosphate", we mean phosphates derived from monobasic orthophosphate,
dibasic orthophosphate, or from orthophosphoric acid. The inorganic
phosphates may be for example an alkali metal or alkaline earth metal
orthophosphate, pyrophosphate, metaphosphate such has hexametaphosphate,
tripolyphosphate, polyphosphate, and combinations thereof. The inorganic
phosphates include for example sodium pyrophosphate, sodium
hexametaphosphate, sodium polyphosphate, potassium pyrophosphate,
potassium hexametaphosphate, or potassium polyphosphate. Preferably, the
inorganic phosphates are alkali metal or alkaline earth metal
orthophosphates, pyrophosphates, hexametaphosphates, or combinations
thereof; and most preferably alkali metal or alkaline earth metal of
pyrophosphates.
The poly(amino acids) useful for inhibiting metal sulfate scale formation
contain amide or peptide bonds as shown below in Formula I.
##STR1##
The peptide linkages are typically formed from the reaction of compounds
which contain a carboxylic acid group and an amino or ammonium group. For
example, the poly(amino acids) may be formed from the reaction of one or
more compounds selected from amino acids, amic acids, ammonium salts of
monoethylenically unsaturated dicarboxylic acids, ammonium salts of
hydroxypolycarboxylic acids, or combinations thereof. Optionally,
additional monomers may be reacted with the compounds used to form the
poly(amino acids).
The term "poly(amino acids)," is meant to include hydrolyzed and
non-hydrolyzed poly(amino acids). "Hydrolyzed polyamino acids" are
anhydropolyamino acids which have been reacted or hydrolyzed with at least
one common base or acid.
The term "poly(amino acids)" as herein defined is also meant to include
homopolymers of amino acids and copolymers of amino acids.
By "homopolymers of amino acids" we mean that the poly(amino acids) have
only one type of repeating unit, where the repeating unit is derived from
the reaction of at least one compound. For example, a homopolymer of
aspartic acid, poly(aspartic acid), may be formed from the reaction of
either aspartic acid, maleamic acid, ammonium salts of maleic acid, or
ammonium salts of malic acid. Poly(aspartic acid), for example, may also
be formed from the reaction of aspartic acid and maleamic acid, or
aspartic acid and ammonium salts of maleic acid.
By "copolymers of amino acids" we mean that the poly(amino acids) contain
at least two different types of repeating units where the repeating units
are derived from the reaction of at least two different compounds. This
definition of copolymer includes copolymers of two amino acids, provided
that the repeating units formed when the two amino acids are reacted are
not the same. For example, a copolymer of aspartic acid and histidine may
be formed from the reaction of aspartic acid and histidine. However, a
copolymer is not formed when the at least two different compounds reacted
produce the same repeating unit. For example, when maleamic acid and
aspartic acid are thermally condensed, the poly(amino acid) formed is a
homopolymer of aspartic acid.
The poly(amino acids) may also be random, sequential, or block polymers. By
"sequential" we mean the repeating units are alternated in a pattern
within the polymer. By "block" we mean that the same type of repeating
units are connected adjacently together in groups within the polymer.
The poly(amino acids) are synthesized by techniques well known to those
skilled in the art. For example, they may be synthesized by naturally
occurring biochemical processes or by synthetic chemical processes.
Suitable processes, for example, are disclosed in "The Peptide Bond" in
The Peptides: Analysis, Synthesis, Biology, edited by E. Gross and J.
Meienhofer, published by Academic Press, NY, Vol 1, pages 1-64 (1979). A
preferred method for synthesizing the poly(amino acids) is disclosed in
U.S. Pat. No. 5,319,145. U.S. Pat. No. 5,319,145 discloses a condensation
reaction method for preparing poly(amino acids). The process utilizes heat
and mild agitation to condense and polymerize the amino acids, amic acids,
ammonium salts of monoethylenically unsaturated dicarboxylic acids, and
optional additional monomers.
The condensation reaction typically proceeds by polymerizing these
compounds to form an anhydropoly(amino acid) by driving off the water
formed from intermolecular condensation of these compounds as well as from
internal cyclization. Water liberated is removed during the reaction to
drive the reaction toward completion.
The condensation reaction may also be conducted in the presence of an acid
catalyst such as for example, orthophosphoric acid and polyphosphoric
acid. When an acid catalyst is used, the acid catalyst is typically added
to the compounds to form a reaction mixture, and the reaction mixture is
heated and agitated to form the anhydropoly(amino acid).
The anhydropoly(amino acid) which results from the condensation may be
further reacted to form a hydrolyzed poly(amino acid). The hydrolysis
reaction is conducted according to techniques well known to those skilled
in the art such as with at least one common base or at least one common
acid to form the corresponding water soluble salt or acid of the
poly(amino acid). Preferably, the hydrolysis may be completely or
partially carried out with any common alkali metal base, alkaline earth
metal base, ammonium hydroxide, or low quaternary salt hydroxide, or
combinations thereof to form the corresponding water soluble salt.
The weight average molecular weight (Mw), of the poly(amino acids) may be
from 1000 to 100,000, preferably 2000 to 30,000, most preferably 3000 to
20,000 as determined by aqueous gel permeation chromatography (GPC) using
as a standard 4500 Mw poly(acrylic acid).
Amino acids, which may be reacted to form the poly(amino acids) include for
example, glycine, alanine, valine, leucine, isoleucine, phenylalanine,
tyrosine, tryptophan, serine, threonine, aspartic acid, glutamic acid,
asparagine, glutamine, lysine, arginine, histidine, methionine, cystine,
cysteine, proline, hydroxyproline, .beta.-alanine, phosphoserine,
hydroxylysine, ornithine, citrulline, homocysteine, cystathionine,
4-aminobutyric acid, or combinations thereof. Preferably, the poly(amino
acid) is formed from the reaction of at least one amino acid selected from
glycine, alanine, leucine, phenylalanine, tyrosine, tryptophan, aspartic
acid, glutamic acid, lysine, arginine, histidine, serine, .beta.-alanine,
4-aminobutyric acid, or combinations thereof. More preferably, the
poly(amino acid) is formed from the reaction of at least one amino acid
selected from aspartic acid, lysine, arginine, histidine, 4-aminobutyric
acid, phenylalanine, or combinations thereof.
Amic acids which may be reacted to form the poly(amino acids) are the
monoamides of monoethylenically unsaturated dicarboxylic acids. Suitable
amic acids include for example the monoamides derived from ammonia or
primary amines, and the acid anhydride, ester or acyl halide of
monoethylenically unsaturated dicarboxylic acids. Preferably, the amic
acids are maleamic acid (the monoamide of maleic acid),
methylenesuccinamic acid (the monoamide of itaconic acid), methylene
glutaramic acid or the monoamides of mesaconic acid, methylenemalonic
acid, fumaric acid, citraconic acid, aconitic acid, alkylmaleic acids,
alkenylsuccinic acids, or combinations thereof. The most preferred amic
acids are maleamic acid, methylenesuccinamic acid, or combinations
thereof.
Ammonium salts of monoethylenically unsaturated dicarboxylic acids which
may be reacted to form the poly(amino acids) are the partial or complete
ammonium salts of monoethylenically unsaturated dicarboxylic acids.
Suitable ammonium salts of monoethylenically unsaturated dicarboxylic
acids include the partial or complete ammonium salts of maleic acid,
itaconic acid, mesaconic acid, methylenemalonic acid, fumaric acid,
citraconic acid, aconitic acid, alkylmaleic acids, alkenylsuccinic acids
or combinations thereof. The preferred ammonium salts of monoethylenically
unsaturated dicarboxylic acids are the ammonium salts of maleic acid.
Ammonium salts of hydroxypolycarboxylic acids which may be reacted to form
the poly(amino acids) are the partial or complete ammonium salts of
hydroxypolycarboxylic acids having at least one hydroxy group and two or
more carboxylic acid groups. Suitable ammonium salts of
hydroxypolycarboxylic acids include for example the ammonium salts of
citric acid, isocitric acid, mucic acid, tartaric acid, or malic acid.
Preferred ammonium salts of hydroxypolycarboxylic acids are the ammonium
salts of citric acid or
Optional additional monomers may be reacted with the compounds used to form
the poly(amino acids). Optional monomers include for example carboxylic
acids, hydroxycarboxylic acids, alcohols, alkoxylated alcohols, amines,
alkoxylated amines, lactones, or lactams, or combinations thereof.
Carboxylic acids useful as optional additional monomers have at least one
carboxylic acid group and may be saturated or ethylenically unsaturated.
Suitable carboxylic acids include for example formic acid, acetic acid,
propionic acid, butyric acid, valeric acid, lauric acid, palmitic acid,
stearic acid, behenic acid, oleic acid, capric acid, linoleic acid,
linolenic acid, sorbic acid, myristic acid, undecanoic acid, naturally
occuring fatty acid mixtures such as for example C.sub.12 to C.sub.14 or
C.sub.16 to C.sub.18 fatty acid mixtures, acrylic acid, or methacrylic
acid or combinations thereof. Additional suitable carboxylic acids are
carboxylic acids having more than one carboxylic acid group such as oxalic
acid, adipic acid, fumaric acid, maleic acid, itaconic acid, aconitic
acid, succinic acid, malonic acid, suberic acid, azelaic acid, furan
dicarboxylic acid, phthalic acid, terephthalic acid, diglycolic acid,
glutaric acid, 1,2,3-propanetricarboxylic acid,
1,1,3,3-propanetetracarboxylic acid, 1,3,3,5-pentanetetracarboxylic acid,
1,1,2,2-ethanetetracarboxylic acid, or 1,2,3,4-butanetetracarboxylic acid
or combinations thereof. Anhydrides of carboxylic acids may also be used
such as for example succinic anhydride, dianhydride of
butanetetracarboxylic acid, phthalic anhydride, acetylcitric anhydride,
maleic anhydride, itaconic anhydride, or aconitic anhydride.
The hydroxycarboxylic acids have at least one hydroxy group and at least
one carboxylic acid group. Suitable hydroxycarboxylic acids include for
example citric acid, isocitric acid, mucic acid, tartaric acid,
hydroxymalonic acid, lactic acid, or malic acid. Additional
hydroxycarboxylic acids include for example glyceric acid,
bis(hydroxymethyl)propionic acid, or gluconic acid.
Alcohols useful as optional additional monomers are monohydric alcohols or
polyols. Monohydric alcohols include for example methanol, ethanol,
n-propanol, isopropanol, butanol, pentanol, hexanol, cyclohexanol,
octanol, decanol, palmityl alcohol, or stearyl alcohol. Polyols include
for example ethylene glycol, glycerol, oligoglycerol, erythritol,
pentaerythrital, sorbital, triethanolamine, polysaccharide, or polyvinyl
alcohol.
The alcohols may also be added to C.sub.2 to C.sub.4 alkylene oxides to
form alkoxylated monohydric alcohols or polyols. For example alkoxylated
polyols such as poly(ethylene glycol), poly(propylene glycol), or
ethoxylated glycerol may be used as optional monomers.
Amines may also optionally be reacted. Amines include monoamines or
polyamines. Suitable monoamines include for example C.sub.1 -C.sub.22
alkyl or aryl amines such as methylamine, ethylamine, butylamine,
diethylamine, cyclohexylamine, octylamine, stearyl amine, oleyl amine and
palmitylamine, hydroxylamines such as N-(carboxymethyl)-hydroxylamine,
N,N-di(carboxymethyl)hydroxylamine, tricarboxymethylhydroxylamine,
ethanolamine, or diethanolamine. Polyamines include for example
ethylenediamine, diethylenetriamine, triethylenetetraamine,
hexamethylenediamine, diaminobutane, histamine, or polyvinylamine. The
amines may also be added to C.sub.2 to C.sub.4 alkylene oxides to form
alkoxylated amines.
As stated previously, the poly(amino acids) added with the inorganic
phosphates to the aqueous system include copolymers of amino acids and
homopolymers of amino acids. Preferably the poly(amino acids) added to the
aqueous system are homopolymers.
Preferred homopolymers of amino acids useful in the present invention are a
reaction product of at least one compound selected from aspartic acid,
glutamic acid, lysine, arginine, histidine, alanine, .beta.-alanine,
4-aminobutyric acid, maleamic acid, ammonium salts of maleic acid, or
ammonium salts of malic acid. More preferably the homopolymers are a
reaction product of at least one compound selected from aspartic acid,
glutamic acid, lysine, maleamic acid, or the ammonium salts of maleic
acid.
Preferred copolymers of amino acids useful in the present invention are a
reaction product of at least one first amino acid and at least one second
amino acid. The preferred copolymers of amino acids have a molar ratio of
the first amino acid to the second amino acid of from 1:99 to 99:1;
preferably from 40:60 to 95:5; and more preferably from 70:30 to 95:5.
The first amino acid of the preferred copolymer is preferably selected from
aspartic acid and glutamic acid. More preferably the first amino acid is
aspartic acid.
The second amino acid of the preferred copolymer is preferably selected
from the group consisting of glycine, alanine, valine, leucine,
isoleucine, phenylalanine, tyrosine, tryptophan, serine, threonine,
asparagine, glutamine, lysine, arginine, histidine, methionine, cystine,
cysteine, proline, hydroxyproline, .beta.-alanine, phosphoserine,
hydroxylysine, ornithine, citrulline, homocysteine, cystathionine, and
4-aminobutyric acid, or combinations thereof. More preferably, the second
amino acid is selected from glycine, alanine, leucine, threonine,
isoleucine, phenylalanine, lysine, arginine, histidine, tyrosine, serine,
threonine, or combinations thereof, and most preferably, selected from
lysine, arginine, histidine, or combinations thereof.
In another embodiment of the present invention, we have discovered that
certain preferred poly(amino acids) are surprisingly effective in
inhibiting the formation metal sulfates in an aqueous system, with or
without inorganic phosphates being present in the aqueous system.
The certain preferred poly(amino acids) contain at least a first compound
selected from tyrosine, tryptophan, histidine or arginine. Preferably the
first compound is histidine.
The certain preferred poly(amino acids) may also contain one or more second
compounds selected from amino acids, amic acids, ammonium salts of
monoethylenically unsaturated dicarboxylic acids, ammonium salts of
hydroxypolycarboxylic acids, or optional additional monomers. Preferably
the one or more second compounds are selected from aspartic acid, glutamic
acid, lysine, glycine, alanine, leucine, phenylalanine, serine,
.beta.-alanine, 4-aminobutyric acid, maleamic acid, ammonium salts of
maleic acid, or ammonium salts of malic acid, or combinations thereof;
more preferably selected from aspartic acid, glutamic acid, or lysine; and
most preferably is aspartic acid.
The certain preferred poly(amino acids) contain from 1 to 100 mole percent;
preferably 1 to 60 mole percent; and most preferably from 5 to 25 mole
percent of the first compound. The certain preferred poly(amino acids)
contain from 0 to 99 mole percent; preferably from 40 to 99 mole percent;
and most preferably from 75 to 95 mole percent of the one or more second
compounds.
These certain preferred poly(amino acids) may be synthesized as described
previously for the poly(amino acids).
The total concentration of the certain preferred poly(amino acids) added to
the aqueous system is that concentration needed to effectively inhibit the
formation of metal sulfate scale. The concentration required generally
depends upon the such variables as pH, temperature, or composition of the
aqueous system to be treated. However, typically, the concentration may be
greater than 0.1, preferably 1 to 5,000, and most preferably 3 to 1000
mg/l.
The scale inhibitors useful in the present invention are effective in
inhibiting the scale formation of metal sulfates for example iron sulfate,
manganese sulfate, and alkaline earth metal sulfates such as magnesium
sulfate, calcium sulfate, barium sulfate, strontium sulfate, and radium
sulfate. The scale inhibitor useful in the present invention is
particularly effective in inhibiting the formation of calcium sulfate and
barium sulfate.
The scale inhibitors are effective in inhibiting the formation of metal
sulfate scale in the presence of other cations and anions. Cations which
may be present in the aqueous system include for example alkali and
alkaline earth metals. Anions which may be present in the aqueous system
include for example carbonate, phosphate, molybdate, phosphonate, oxalate,
and hydroxide ions. These cations and anions may combine to produce
insoluble salts such as for example calcium carbonate, iron oxide, calcium
phosphate, zinc hydroxide, iron molybdate, and calcium phosphonate.
The scale inhibitors useful in the present invention are more effective in
inhibiting the formation of metal sulfate scale in aqueous systems having
a pH from 2 to 14; preferably, from 3 to 9. Of particular importance are
scale inhibitors useful in the present invention which are effective for
inhibiting sulfate scale formation at pH's less than 7 where the metal
sulfate is typically less water soluble.
The scale inhibitors are more effective in aqueous systems where the
temperature is from 5.degree. to 250.degree. C.; preferably 15.degree. to
95.degree. C.
Other additives may be added to the aqueous system in addition to the scale
inhibitors useful in the present invention. The other additives added will
depend on the type of aqueous system. However, common other additives
include, for example, one or more corrosion inhibitors, metal
deactivators, additional scale inhibitors, threshold agents, and
precipitating agents
Corrosion inhibitors which may be added to the aqueous system include for
example water soluble zinc salts, phosphates, polyphosphates, phosphonic
acids, nitrates, molybdates, tungstates, silicates, ethanolamines, fatty
amines, and poly(carboxylic acids).
Metal deactivators which may be added to the aqueous system include for
example benzotriazole, or bis benzotriazole, derivatives of benzotriazole
or tolyltriazole.
Additional scale inhibitors include poly(acrylic acid),
phosphino-poly(carboxylic acids), hydrolyzed poly(acrylonitrile),
poly(methacrylic acid), poly(maleic acid), poly(acrylamide), and
copolymers of acrylic acid, methacrylic acid, acrylamide, acrylamide
propionic sulfonic acid, acrylamido methylpropane sulfonic acid, alkyl
acrylamide, styrene, and maleic acid.
Threshold agents which may be added to the aqueous system include, for
example, 2 phosphonobutane-1,2,4-tri-carboxylic acid,
hydroxyethyl-diphosphonic acid, hydrolyzed poly(maleic anhydride), and
hydroxyphosphonoacetic acid. Precipitating agents which may be added
include alkali metal carbonates.
Other additives which may be added to the aqueous system include for
example, oxygen scavengers, sequestering agents, antifoaming agents, and
biocides.
An added advantage to the scale inhibitors useful in the present invention
is that they may also be effective as corrosion inhibitors. Thus, the
aqueous system treated with the scale inhibitors useful in the present
invention may not need to be treated with a separate corrosion inhibitor.
The scale inhibitor useful in the present invention may also be effective
in inhibiting the formation of other scales such as calcium carbonate,
calcium phosphonate, iron oxide, and zinc phosphate, calcium phosphate.
The aqueous systems in which the scale inhibitors may be added are any
aqueous system where the scale formation of metal sulfate have a
deleterious effect on the performance of the aqueous system. Aqueous
systems include for example cooling water systems, boilers, heat exchange
equipment, reverse osmosis equipment, geothermal systems, oil and gas
production operations, sugar production operations, flash evaporators,
desalination plants, paper making equipment, and steam power plants.
Preferably the aqueous system is for example reverse osmosis equipment,
oil and gas production operations, desalination plants, and paper making
equipment. The scale inhibitor may be added to various parts of the
aqueous system where ever scale formation of sulfate is a problem.
Some embodiments of the invention will now be described in detail in the
following Examples. In all examples where a weight average molecular
weight (Mw) is reported, the Mw was measured by gel permeation
chromatography using 4500 Mw poly(acrylic acid) as a standard. The
abbreviations used in Tables 2-4 are defined in Table 5.
The scale inhibitors useful in the present invention were tested for their
ability to inhibit barium sulfate scale formation in Examples 1-29. The
test method used in Examples 1-29 for measuring inhibition of barium
sulfate consisted of the following steps: 1) Preparation of test solutions
containing the scale inhibitor, 2) Incubation of the test solutions, and
3) Measurement of barium which did not precipitate in the test solutions.
Accordingly, in comparing two test solutions, the test solution having the
higher percent barium sulfate inhibition, contains a scale inhibitor which
is more effective in inhibiting metal sulfate scale formation.
The test solutions were prepared from a barium containing solution, a
sulfate containing solution, a buffer solution, and an inhibitor solution
containing the inhibitor. The composition of the barium and sulfate
containing solutions were similar to the following solutions shown in
Table 1:
TABLE 1
______________________________________
Composition of Barium and Sulfate Containing Solutions Similar to
those Actually Used*
Concentration (mg/l)
Components Barium Solution
Sulfate Solution
______________________________________
KCl 709 878
NaCl 74,202 23,953
CaCl.sub.2 7,778 1,186
MgCl.sub.2 1,974 5358
BaCl.sub.2 382 0
SrCl.sub.2 1,038 14
Na.sub.2 SO.sub.4
0 4,378
NaHCO.sub.3 683 171
Deionized Water
balance balance
______________________________________
*The barium and sulfate containing solutions shown in Table 1, when
combined with the buffer and inhibitor solutions according to the test
method described herein to form an inhibitor test solution provide the
same inhibitor test solution composition as was used in Examples 1-29.
The composition of the buffer and inhibitor solutions were as follows:
______________________________________
Components Concentration
______________________________________
Buffer Solution
CH.sub.3 COONa.3H.sub.2 O
13.6 g/100 ml
H.sub.3 CCOOH 0.535 g/100 ml
Deionized Water balance
Inhibitor Solution
Scale inhibitor to be tested for
1 g/l
BaSO.sub.4 inhibition.
Deionized Water balance
______________________________________
The barium and sulfate containing solutions were filtered through a 0.45
micron filter and adjusted to a pH of 6.0 with dilute HCl. The inhibitor
solution was adjusted to a pH of 6 with dilute HCl or dilute NaOH.
The test solutions containing a scale inhibitor, hereinafter called the
"inhibitor test solution" were prepared by combining 1 ml of the buffer
solution, 50 ml of the sulfate containing solution, the desired amount of
inhibitor solution, and 50 ml of the barium containing solution. In the
Examples where the test solution contained a combination of an inorganic
phosphate and polymer as the scale inhibitor, two inhibitor solutions were
prepared: 1) a solution containing 1 gram per liter of inorganic
phosphate, as PO.sub.4, and 2) a solution containing 1 gram per liter of
poly(amino acid), as polymer in the acid form. The two scale inhibitor
solutions were then added to the test solution to produce the desired
concentration of scale inhibitor.
As controls, a no inhibitor test solution, a sulfate test solution and a
barium test solution were prepared. The no inhibitor test solution was
prepared by combining 1 ml of the buffer solution, 50 ml of the sulfate
containing solution, 50 ml of the barium containing solution, and
deionized water in an amount equal to the amount inhibitor solution added
to the inhibitor test solution. For example, if 2.4 ml of inhibitor
solution (total) was added to the inhibitor test solution, 2.4 ml of
deionized water was added to the no inhibitor test solution. The sulfate
test solution was prepared by combining 1 ml of the buffer solution, 100
ml of the sulfate solution, and deionized water in an amount equal to the
amount of inhibitor solution added to the inhibitor test solution. The
barium test solution was prepared by combining 1 ml of the buffer
solution, 100 ml of the barium containing solution, and deionized water in
an amount equal to the amount of inhibitor solution added to the inhibitor
test solution.
The inhibitor, no inhibitor, sulfate, and barium test solutions were placed
in a water bath at 85.degree. C. and gently shaken for 24 hours. After the
24 hour incubation period, the test solutions were removed one at a time
from the water bath and a diluted test solution was prepared from each
test solution for analyzing barium content. The diluted test solution was
prepared by adding to a 100 ml flask the following ingredients in the
order listed:
1) 5 ml of the EDTA Solution
2) 30 ml of deionized water
3) 5-10 g of supernatant taken from the incubated test solution
4) deionized water (balance to make 100 ml)
The EDTA Solution consisted of 100 grams per liter of K.sub.2
EDTA.cndot.2H.sub.2 O, and deionized water (balance). The pH of the EDTA
Solution was adjusted to 10.5 with KOH pellets.
The diluted test solutions were measured for barium using direct current
plasma on a Spectra Span 7 DCP Spectrometer manufactured by Applied
Research Laboratories Fisons located in Valencia, Calif. The concentration
of the barium in the undiluted test solutions was calculated from the
measured values of barium. The percent barium sulfate inhibition was
obtained from Formula II:
##EQU1##
where: Ba Inhibitor=concentration of barium in inhibitor test solution
Ba No Inhibitor=concentration of barium in no inhibitor test solution
Ba Barium=concentration of barium in barium test solution
Ba Sulfate=concentration of barium in sulfate test solution
The scale inhibitors shown in Table 2 were tested for barium sulfate
inhibition. Table 2 shows that homopolymers of amino acids added with
inorganic phosphates in an aqueous system are more effective in inhibiting
the formation metal sulfate scale than homopolymers of amino acids alone
or inorganic phosphates alone.
Table 2 shows that when a homopolymer of aspartic acid (polyaspartic acid)
is combined with orthophosphate, pyrophosphate or hexametaphosphate in a
test solution (Examples 5-7, 10-11, 13-14, and 16-17) the polyaspartic
acid and the inorganic phosphate inhibit the formation of barium sulfate
more effectively than polyaspartic acid alone at a given Mw (Comparative
Examples 4, 8, 12, and 15) or the phosphate alone (Comparative Examples
1-3). The improvement observed is particularly significant when
polyaspartic acid is added with pyrophosphate to a test solution.
The polyaspartic acid in Examples 4-7 was prepared from the reaction of
maleic anhydride and ammonia to form polysuccinimide. A 30 weight percent
aqueous ammonia solution was used as a diluent in the reaction. The mole
ratio of ammonia to the maleic anhydride used was 1.05 moles ammonia to 1
mole maleic anhydride. The diluent was used at a mole ratio of 0.13 moles
ammonium hydroxide to 1 mole maleic anhydride. The polysuccinimide formed
from the reaction was then hydrolyzed at an aqueous pH of 10.8 at
90.degree. C. for 30 minutes with sodium hydroxide to form the
poly(aspartic acid). In the preparation process the maleic anhydride and
ammonia reacted to form an ammonium salt of maleic acid, which then
condensed to form the polysuccinimide.
The polyaspartic acid in Examples 8-17 was prepared from the thermal
condensation reaction of a mixture of aspartic acid and 85 weight percent
orthophosphoric acid to form poly(succinimide). The mixture of the
aspartic acid and the othophosphoric acid was varied to achieve the Mws
reported in Table 2. The mixtures were the following:
Examples 8-11
97.5 weight percent aspartic acid, and
2.5 weight percent of the orthophosphoric acid
Examples 12-14
90 weight percent aspartic acid, and
10 weight percent of the orthophosphoric acid
Examples 15-17
80 weight percent aspartic acid, and
20 weight percent orthophosphoric acid
After the condensation reaction was complete in Examples 8-17, the
orthophosphoric acid was washed from the polysuccinimide using water to
completey remove the acid. The polysuccinimide in Examples 8-17 was then
hydrolyzed at a pH of 10.8, at 90.degree. C., for 30 minutes, using sodium
hydroxide to form the poly(aspartic acid).
TABLE 2
______________________________________
Inhibition of Sulfate Scale Formation with Homopolymers of
Amino Acids and Phosphates
Polymer
Phosphate
Mw of Conc. Conc. % BaSO4
Example
Scale Inhibitor
Polymer.sup.1
(mg/l).sup.2
(mg/l).sup.3
Inhibition
______________________________________
1 (comp.)
ORTHO -- 0 4.8 1.1
1a (comp.)
ORTHO -- 0 25 4.1
2 (comp)
PYRO -- 0 4.8 0.0
2a (comp)
PYRO -- 0 25 7.1
3 (comp)
SHMP -- 0 4.8 3.5
3a (comp)
SHMP 0 25 136.7
4 (comp)
PASP 1930 24 0 3.8
5 PASP/ORTHO 1930 24 4.8 5.4
6 PASP/PYRO 1930 24 4.8 103.7
7 PASP/SHMP 1930 24 4.8 5.8
8 (comp)
PASP 5330 24 0 8.8
9 PASP/ORTHO 5330 24 4.8 5.3
10 PASP/PYRO 5330 24 4.8 106
11 PASP/SHMP 5330 24 4.8 75.7
12 (comp)
PASP 8000 24 0 13.0.sup.4
13 PASP/ORTHO 8000 24 4.8 27.1
14 PASP/PYRO 8000 24 4.8 103.5.sup.4
15 (comp)
PASP 16,400 24 0 13.7.sup.4
16 PASP/ORTHO 16,400 24 4.8 49.9
17 PASP/PYRO 16,400 24 4.8 107.4.sup.4
______________________________________
.sup.1 Mw of amino acid homopolymer
.sup.2 Concentration of polymer, as polymer in acid form, in scale
inhibitor test solution
.sup.3 Concentration of phosphate, as PO.sub.4, in scale inhibitor test
solution
.sup.4 Average of 2 data points
Copolymers of amino acids with and without inorganic phosphates were tested
for their ability to inhibit barium sulfate scale formation. The ability
of copolymers of amino acids to inhibit barium sulfate with and without an
inorganic phosphate is shown in Table 3.
Table 3 demonstrates that a combination of copolymers of amino acids and
inorganic phosphates are more effective in inhibiting barium sulfate scale
formation (Examples 19, 21, 23, 25) than using the copolymers or the
inorganic phosphates alone.
Examples 18 and 20 in Table 3 show that a copolymer of aspartic acid and
histidine (a basic amino acid) or a copolymer of aspartic acid and
tyrosine (an aromatic amino acid) are particularly effective in inhibiting
barium sulfate scale formation when compared to poly(aspartic acid)
(Examples 4, 8, 12, and 15, Table 2) and other copolymers of amino acids
(Comparative Examples 22, 24, 26, 27, 28). The copolymer of aspartic acid
and histidine in Table 3 is the most effective for inhibiting barium
sulfate scale formation in comparison to the other copolymers shown in
Table 3.
The copolymers in Table 3 were prepared from the thermal condensation of
amino acids. For each copolymer in Table 3 (Examples 18-28), the amino
acids were thermally condensed in the proportions of 80 moles of the first
amino acid shown in Table 3, under "Scale Inhibitor" (i.e., aspartic acid)
to 20 moles of the second amino acid shown in Table 3 to form a reaction
product. In Example 27, the proportions of amino acids condensed were 80
moles of aspartic acid to 20 moles of serine; however, a reaction product
of aspartic acid, serine, and d-alanine formed in the proportions shown in
Table 3. In Examples 18-28, the thermal condensation reaction was
performed using an acid catalyst. Polyphosphoric acid was used in Examples
18-25 and 27.degree.-28, and orthophosphoric acid was used in Example 26.
After the reaction was complete, the acid catalyst was completely removed
from the reaction product by washing with water. The reaction product was
purified by dialysis, and then hydrolyzed at a pH of 10.8, with sodium
hydroxide, at 90.degree. C. over 30 minutes.
TABLE 3
______________________________________
Inhibition of Sulfate Scale with Copolymers of Amino Acids/
with and without Phosphates
Polymer
Phosphate
Polymer Conc. Conc. % BaSO4
Example
Scale Inhibitor
Mw (mg/l).sup.5
(mg/l).sup.6
Inhibition
______________________________________
18 87.7 ASP/ 4360 28.8 0 33.0
12.3 TYR
19 87.7 ASP/ 4360 24.0 4.8 83.6
12.3 TYR
20 90.0 ASP/ 3420 28.8 0 79.4
10.0 HISHCl
21 90.0 ASP/ 3420 24.0 4.8 98.1
10.0 HISHCl
22 (comp)
85.4 ASP/ 5350 28.8 0 7.6
14.6 GLY
23 85.4 ASP/ 5350 24.0 4.8 101
14.6 GLY
24 (comp)
84.6 ASP/ 3190 28.8 0 7.8
15.4 PHE
25 84.6 ASP/ 3190 24.0 4.8 98.2
15.4 PHE
26 (comp)
85.1 ASP/ 5700 28.8 0 15.3
14.9 LYS
27 (comp)
95.9 ASP/0.07
5660 28.8 0 19.8
SER/4.02 d-ALA
28 (comp)
87.1 ASP/ 2980 28.8 0 10.1
12.9 LEU
______________________________________
.sup.5 Concentration of polymer, as polymer in acid form, in scale
inhibitor test solution
.sup.6 Phosphate tested was sodium pyrophosphate, concentration shown is
as PO.sub.4
A combination of poly(aspartic acid) of molecular weight 7970 (made by the
same procedure as in Examples 12-14) and sodium pyrophosphate was tested
at various concentrations for inhibition of barium sulfate scale
formation. The results are shown in Table 4. Comparative Example 31 and
Example 32 demonstrate that a scale inhibitor of 0.5 mg/l pyrophosphate
and 5 mg/l of poly(aspartic acid) is more effective in inhibiting barium
sulfate scale formation than 5 mg/l of poly(aspartic acid) alone. The
percent improvement between Examples 31 and 32 is 129% based on Example
31. Example 33, demonstrates that a scale inhibitor of poly(aspartic
acid), dosed at a concentration of 1 mg/l, and pyrophosphate, dosed at a
concentration of 10 mg/l, is more effective than pyrophosphate alone dosed
at 25 ppm (Example 2a) and poly(aspartic acid) dosed at 5 mg/l
(Comparative Example 31).
TABLE 4
______________________________________
Effect of Polymer and Phosphate Concentration on Sulfate Inhibition
Scale
Polymer.sup.7
Phosphate.sup.8
Concentration
Concentration
% BaSO4
Example (mg/l) (as PO4, mg/l)
Inhibition
______________________________________
12 (comp) 24 0 13.0
30.sup.9 24 4.76 103.5
31.sup.9 (comp)
5 0 -7.4
32.sup.9 5 0.5 2.2
2a (comp) 0 25 7.1
33.sup.9 1 10 102.4
______________________________________
.sup.7 Polymer tested in Examples 30-33 was poly(aspartic acid) having a
Mw of 7970.
.sup.8 Phosphate tested was sodium pyrophosphate
.sup.9 The Barium Solution added to the test solution for Examples 30, 32
and 33 was diluted to reduce the concentration of the components in the
barium containing solution in half.
TABLE 5
______________________________________
Abbreviations Used in Tables 2-4
Abbreviation Definition
______________________________________
comp comparative
ORTHO orthophosphoric acid
PYRO sodium pyrophosphate
SHMP sodium hexametaphosphate
PASP poly(aspartic acid)
ASP mole percent aspartic acid
TYR mole percent tyrosine
HISHCI mole percent histidine hydrochloride
GLY mole percent glycine
PHE mole percent phenylalanine
LYS mole percent lysine
SER mole percent serine
d-ALA mole percent d-alanine
LEU mole percent leucine
______________________________________
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